exhaust in cuddy
 

I have a 98 32. By recommendation, I put 4 inch, 90 degree rubber downturns on the exhaust tips to mitigate some of the hull vibration at idle. They worked awesome, BTW. Also, I had a riser give out a couple of weeks ago, so i have new-ish stock merc exhaust on one motor. This is the first weekend that I have had it that I have really run it ballz out for an extended amount of time.
I have noticed that now, after a run (even a short one) that the cuddy smeels very stongly of exhaust. Enough that someone would probably pass out if they sat down there with it closed up. I have rechecked my work on the manifolds, and no leaks on those, or the various tubes and pipes going to the silent choice through the hull. I don't see how the downturns could be making any more back pressure than the pipes straight out, and even if they were, where would it be coming from? Most of the exhaust joints for the risers to the through hull are all wet joints, so I should be seeing water coming out as well. I know that the gasses are pushing up the drain tubes through the step and into the cuddy, so it's not a question of how it's getting up there, more of where is it coming from. Ay ideas would be greatly appreciated.

I'd suggest taking the down tubes off and running the boat to confirm the condition exists with or without the down tubes. With the down tubes fumes may be trapped under the platform and are somehow sucked back into the boat and migrating into the cabin.
ed

That's where I'm going to start. I was really hoping to keep them on there, just because of the vibration issue, but not worth it if that's what's causing the issue. I'm going to pull them and try it next weekend.

Do you still have the large vent holes on the back of the boat? (edited, see below post)

Be VERY careful with this. I'd take a battery powered CO detector with you. It's probably obvious that it's CO; but the CO detector will tell when the PPM are too dangerous to be near.
A member here on OSO had a very bad CO scare with his boat a few months back; damn near killed him.

do NOT tape those holes shut ...they are the vent holes for the blowers...

My bad if they are the blower exits, but I've seen others when they holes have been removed, especially on the 32.

Could the blowers be wired backwards; sucking air in--exhaust?

It does it with the blowers off and on, and only under power...it seems I can idle in and out of the marina all day long without being able to replicate the issue. And this is the first weekend that it has happened. Like I said earlier, the only 2 things that have changed are the downturns, and the manifolds, and I can't find any leaks in the exhust system inside the boat.

Its probably traveling up the gunwales somewhere. I don't the exacts of the Fountain layout, but can you find some way to block off the gunwale inner air pathways at the bulkhead? I wonder if the Fountain depends on this for fresh air for the engines. Anyone know?

That's about all I can comee up with too. I know there are some drains somewhere that help move water in from ot the step, but they are under the floor. I thought about trying to plug up the 2 small holes between the engine bulkhead, but I probably am better off trying to figure out where it's coming from in the first place. I rechecked the manifold bolts one the one that I replaced, and they were still tight. I kinda figured that if I had a manifold leak, I would be a ble to hear it pretty well.

Do you have any rubber hoses between your manifolds and tailpipes? No leaks there? Rope gaskets?

Are your tailpipes sealed well as they pass through the transom? With the way those particular Fountains sit in the water, trapping the exhaust gasses under the platform, those gasses want to go somewhere.

Pure speculation here, but when you had just tips, perhaps gasses were escaping "better" out the back of the boat; than now with the rubber tips installed. You could have (CO) air pumping back into your bilge and then forward while off plane; sheerly because they those gasses are "circling" under the platform rather than heading straight aft--reentering the bilge around your tailpipes.

Do you get water in your bilge when idling? You could have water saturated with CO entering your bilge also around the tailpipes; and the CO is moving up to your cabin once the water is in your bilge.

There is a 38 Nortech that was built with virtually no cabin, side by side engines, and a molded in platform similar to the Fountains. It was purely a race boat. When idling, it would build up so much pressure it would actually lift the transom up and burp the CO/exhaust out the back of the boat every 30 seconds.

Just thinking out loud on the keyboard.

Next step might be running the engine(s), with the SWP disabled; (single belt removed, or if a serpentine, remove the impeller; reinstall SWP; and run the engine for just a minute or two)...
...while someone stands at the transom with two Nerf footballs lightly pressed into the tailpipes creating backpressure in the hopes you might find an/the exhaust leak.

That's kind of the direction that I was thinking also. I do have all the rubber hoses needed with the silent choice installed. I have gone back over them and made sure everything was tight. When I replaced the port manifolds, I had to take all of that off, and I did find one of the exhaust tip through hulls was loose. I siliconed the crap out if it and put it back on. I checked the others, and they were still tight. I do get some water entering the bilge, I just can't seem to find where. All the gimbal/shift bellows are less than a year old, so I doubt it's there, I think it might be a through hull for the mechanical tab or drive indicators, I just can't see it laying on top of the motors.
I think you're on to something with the amount of gas secaping under the platform. The downturns helped the vibration considerably, about 80% better, but you are right, those gasses have to go somewhere. Before I put them on, it would blow so much exhaust out from under the swim platform that it would completely soak the back of my sunpad. Now, not at all. You're right, it has to go somewhere.
I'm leaning very much towards it being the downturns. I think I'm going to have to find another option. I saw a formula a few weeks ago that had stainless extensions on the tails that got the exhaust just past the swim platform. I'm not sure where to go to start researching where to get them. I'm thinking an exhaust shop might, but I seriously doubt they have stainless 4" just laying around. I'm going to see if I can maybe find somewhere that has some stout rubber 4" straight pipe that I can get cut to fit and test before I start throwing money at stainless.
I appreciate you brainstorming with me, I'll keep you posted on how it works. I'll drop the downturns off and test this weekend to see if it goes away.

Is the silent choice still installed and functional? Did you try running the exhaust out the outdrive?

I would try that, also if you have silent choice why bother with the down turns?

The silent choice is installed, but inop. The motors for the diverters have quit working, and I haven't decided if I'm going to repair, or just remove completely to get some weight out of the back of the boat. The downturns were just a stopgap measure to get through the season before I had to decide what to do. The boat sits low enough that I will ahve to remove the y pipes and install a block off plate, and I don't have any way to pull the motors myself, so that's an $800 bill per side just to R and R the motors.

Are your eng compartment vents facing the right way (all fwd on one side and all aft on other side)? I could see if you had a exh leak, and the vents were all facing forward, when you got on plane the eng compartment would pressurize and force air from bilge up into gunwales and cabin water drain tube.

I don't have directional vents on the side of the engine comt. It has one big intake on each side, and that's it.

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the silent choice was just too quiet for me at idle,,as I posted on another thread I installed these tips which are actually diesel truck exhaust tips and they extend 13" out making it easier for the exhaust NOT to get trapped under the swim platform(as you can see in the picture I attached) and I think they look good and now I dont have to hit the silent choice switch every time I come to a no wake and I now have a nice sound out of the exhaust at idle

That is a GREAT idea. Thanks for posting that!

if you look closely you can see the exhaust coming out from under the swim platform at idle..

where did you get/order those from? thay might just be the ticket.

here you go

MBRP Exhaust 4 4" Tip Turn Down T5081 | eBay
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awesome idea. I have no freakin idea why I didn't think of that, considering I have a truck with the same exact tip on it. Gotta love this site!